1. Field of the Invention
This invention relates to a catalyst deterioration-determining system for internal combustion engines, which determines deterioration of a catalyst arranged in the exhaust system of the engine, for purifying exhaust gases emitted from the engine.
2. Prior Art
To determine deterioration of a catalyst provided in an internal combustion engine, which purifies exhaust gases emitted from the engine, a catalyst deterioration-determining method is conventionally known, for example, from Japanese Laid-Open Patent Publication (Kokai) No. 5-106494, which controls the air-fuel ratio of an air-fuel mixture supplied to the engine in a feedback manner responsive to an output from an oxygen concentration sensor arranged in the exhaust passage of the engine at a location downstream of a catalyst arranged in the same, measures an inversion period of the output from the oxygen concentration sensor, and determines whether or not the catalyst is deteriorated, based on the measured inversion period.
According to the conventional method, however, the determination of deterioration of the catalyst is carried out by comparing the measured inversion period as it is with a predetermined reference value without contemplating operating conditions of the engine at the measurement. As a result, the measured inversion period contains an error depending on operating conditions of the engine at the measurement. Therefore, the conventional method remains to be improved in the determination accuracy.
Further, according to the conventional method, it is difficult to determine the deterioration degree of the catalyst with high accuracy, and therefore the method still remains to be improved in order to satisfy stricter legal regulations of exhaust gases.